Electronic circuits, and electronic circuit devices, inherently generate heat when they are being used (i.e., when electrical power is being applied to the circuit or device). This heat can reduce the efficiency of the circuit or device, and in certain instances the heat generated by the circuit or device (“circuit”) can be sufficient to damage the circuit, and potentially causing the circuit not to work. It is therefore desirable to find a way to remove this heat from the circuit. Two primary modes of heat removal are used: passive and active. Passive heat removal typically involves the use of heat sinks, which provide a larger surface area for convective transfer of heat away from the circuit. In most passive systems, the heat transfer is by natural convection. Active heat removal typically involves the use of a forced-air system to blow a fluid, such as relatively cool air or a liquid, across the circuit. In many electronic apparatus, such as personal computers, these two types of heat removal methods are combined so that cooling air is forced past a heat sink to thereby provide forced convective heat removal from the heat sink. This latter cooling technique can remove significant quantities of heat from an electronic apparatus.
When cooling a printed circuit assembly (“PCA”), which comprises a printed circuit board (“PCB”) on which circuit devices are mounted, it is generally desirable that the cooling air in a forced convection cooling system blows across the PCB in a direction generally parallel to the plane of the PCB. However, it can be difficult to obtain a good flow of air across the PCB due to (1) the boundary layer effect (i.e., the velocity at the surface of the PCB will be zero, and therefore velocities very near the surface will be low), and (2) structures mounted, to the PCB board can block the flow of air across the PCB.
Another shortcoming of prior art solutions for cooling PCAs arises where a PCA includes several circuit devices attached to the PCB. For example, in a computer having two or more microprocessors mounted to a single PCB, each processor would typically be provided with its own heat sink. This increases the cost of the overall unit versus using a single heat sink to cool multiple IC devices. On some other instances, like on PCA designs where IC devices must be relatively close to each other, the individual heat sink approach may even prove to be unfeasible. However, connecting two or more IC devices with a single heat sink can cause severe stress on the devices due to height and coplanarity difference between the devices, plus the net effect of the differential thermal expansion of the materials and structures comprising the devices when they are considered in stacked accumulation. Further, conventional thermal interfaces (for example, grease, phase change, and/or polymers) can effectively address filling slight surface imperfections and small coplanarity tolerances between mating surfaces, but they can not comply to large or variable gaps expected from this type of application.